Polyethylene terephthalate (PET), or polyester as it is commonly known, has been widely used as a polymeric base material in a variety of applications, including photographic films, beverage containers, and recording tapes. In many of these various applications, the polymeric base is coated with various binders, adhesives, and metal compounds. For example, in photographic films, a polyester base is coated with an adhesive polymer layer formed from copolymers of polyvinylidene chloride and polyvinyl chloride, hereinafter referred to as "PVDC-based resin". The PVDC-based resin layer permits the adhesion of a light-sensitive emulsion layer to the polyester base. In photographic films, this light-sensitive emulsion layer is usually a silver halide dispersed in gelatin. PVDC-based resin is used to bond the emulsion to the polyester surface because gelatin does not bond sufficiently thereto. In other applications, a PVDC-based resin layer may be used in manufacturing polyester soda bottles and other polymeric articles.
Polyester film is being used in increasing amounts for photographic support, magnetic recording tapes, graphic arts materials, electrical insulation, and other applications requiring a clear, strong, dimensionally stable and chemically resistant film. Although other polymeric bases are utilized, polyester has become the standard film for photographic use and, in particular, for that type of film as described above which employs a silver halide dispersed in gelatin.
Although other polymeric adhesives may be utilized, most films presently employ polyvinylidene chloride resin or congeners thereof. PVDC-based resin adheres tightly to both the polyester surface and the silver halide gelatin emulsion. The PVDC-based resin layer is often called the "subbing" layer. Particularly, copolymers of polyvinylidene chloride and polyvinyl chloride or copolymers of polyvinylidene chloride and polyacrylonitrile are generally used as the subbing layer. The polyvinyl halide subbing layers are generally extremely thin, usually on the order of 1 mil in thickness and sometimes as thin as 0.1 mil or less.
Each year, millions of pounds of polyester and other polymeric bases are scrapped because the presence of the polyvinylidene chloride-based coating inhibits recovery of clean recyclable polymer. Due to problems in the manufacturing process, some of this polyester film never receives a silver halide layer. Instead the base is discarded with only the PVDC-based resin layer coated thereon. Moreover, many of pounds of X-ray and other photographic films are scrapped without recovering the valuable silver contained therein. It would, of course, be very desirable to convert scrap polyester into clean polyester and to convert the scrap films containing metals into a form from which the metals may be recovered. Recovery of silver from the halide emulsion provides both reusable silver and reduces solid waste disposal problems, while protecting our natural resources and environment from silver in waste streams.
A traditional method employed to recover silver from used photographic film is to incinerate the film and reclaim the silver from the ash by pyrometallurgical processing. As expected, the economics of this process are not favorable when silver prices are low.
Various other methods have been used in attempts to recover the silver and polymeric base material from both used ("black") and unused ("green") photographic film and scrap PVDC-based resin-coated PET. Many of the prior methods treat scrap photographic film with hot caustic solutions and/or various solvents. For example, U.S. Pat. Nos. 4,602,046 and 4,612,057 to Buser et al. describe methods for recovering silver and polyester from photographic film by employing an alkaline solution and high shear conditions. Likewise, U.S. Pat. No. 5,064,466 to Hilton shows the use of a stripping formulation having alkaline bleach solution in a process for recovering silver. As disclosed therein, the stripped solution containing silver, silver halide and animal protein is then treated with a strong or highly ionized acid resulting in the precipitation of hydrolyzed animal protein, silver and silver halide. U.S. Pat. No. 3,647,422 to Wainer employs a caustic bath solution, various mechanical rolling and scraping treatments, as well as electrolysis to recover silver and polyester from processed photographic film.
U.S. Pat. No. 3,652,466 to Hittel is directed to a process for recovering polyester utilizing a aqueous alkali solution to form a slurry from small pieces of scrap film. A classification column is used to separate the silver halide emulsion layer and the vinylidene chloride copolymer coating from the polyester pieces. Thereafter, the silver compounds are recovered from the emulsion and vinylidene chloride copolymer sludge.
Various solvents and other agents have also been employed to recover silver and polyester. Such agents include oxidizing agents such as potassium permanganate as described in U.S. Pat. No. 3,047,435 to Wemple, monoethanolamine as described in U.S. Pat. No. 3,928,253 to Thornton et al., and various polar aprotic solvents described in U.S. Pat. No. 3,873,314 to Woo et al.
Additionally, U.S. Pat. No. 3,649,250 to Dorenfeld et al. describes the recovery of silver from photographic film by converting the silver to silver cyanide and then recovering silver by the Merrill-Crow process. U.S. Pat. No. 4,392,889 to Grout and U.S. Pat. No. 4,828,717 to DeLeeuw et al. show other apparatus such as baths for recovering silver and/or plastic from photographic film.
A PVDC-based resin was first employed in the 1950's as the subbing layer for adhering silver halides to polyester. Prior to the use of PVDC-based resin to adhere metals to polymeric bases, various reducing agents had been employed to reduce silver to a recoverable form from various silver-containing solutions and silver-containing cellulosic papers. For example, U.S. Pat. No. 1,448,475 to Weisberg discloses the use of an ordinary sugar to precipitate silver from a sodium thiosulfate solution. The silver sodium thiosulfate solution described therein had been created by washing the silver salt from photographic plates. U.S. Pat. No. 2,503,104 to Farber describes the use of a hydrolyzed sugar solution obtained from wood and a caustic solution to reduce and recover silver from various photographic solutions. As described therein, silver is precipitated from spent photographic solution by heating the solution to its boiling point, rendering it alkaline, and reducing the silver by using organic reducing agents. U.S. Pat. Nos. 1,350,157 to Horton and 1,637,990 to Ellis disclose methods for recovering silver and camphor from cellulosic products, including waste photographic film, by treating with an alkali solution and various solvents which cause the silver to be precipitated out.
Although various reducing sugars have been used to recover silver, such sugars were used to reduce silver after it was in a solution, not to recover silver directly from a solid base. None of the previously-mentioned patents employing reducing sugars solved the problem of removing silver halides bound to a polymer-base film by a polyvinylidene halide subbing layer.
One current commercial process for recovering silver and/or polyester from used or unused photographic film employs the following steps: (1) grinding film into small chips; (2) adding an enzyme to the ground film to decompose the gelatin and liberate the silver from the silver halide emulsion; (3) adding sodium borohydride to reduce the silver from the silver halide emulsion after separating the film therefrom; (4) adding a coagulant to coagulate metallic silver and allowing it to settle into a sludge; and (5) roasting the sludge to remove organics and obtain metallic silver. In addition, caustic potash or soda may be added to the enzymatic and reduction reactions to maintain the pH as necessitated by the particular reagents.
This process suffers from the fact that sodium borohydride is very expensive (sodium borohydride is currently priced at about $25.00 per pound), thus rendering the process uneconomical, especially when silver prices are low. Furthermore, the process often results in fires and creates other safety concerns due to its hazardous reaction steps. The process also produces an abnormally large amount of fines from which the PVDC-based resin-adhered silver has not been removed. Fines include small particles of polyester film that have not been stripped of the silver halide. Fines disposal increases costs as well as increases potential for harm to the environment caused by their burning. Moreover, the resulting roasted silver powder is only about 60% to 80% pure silver.
Although various agents and processes are known to separate silver, PVDC-based resin, and polyester, the particular features of the present invention are absent from the art. The prior art is generally deficient in affording a caustic/reducing sugar composition that removes the PVDC-based resin layer while, at the same time, reduces the silver to obtain metallic silver from photographic film. Moreover, the prior art does not suggest a composition or process as described herein for removing PVDC-based resin from non-silver-containing polyester or for reducing metals attached as metal halides to a substrate. The present invention overcomes the shortcomings of the prior art in that the composition and process disclosed herein result in lower processing costs, less hazardous operational procedures, and increased yields and purity of silver recovered from various PVDC-based resin-layered films and other substrates.